Coral reefs, atolls, and islands resultant of the buildup of coral: 1 polyps, 2medusae,3 exoskeleton,4 wastes What one of these causes? I think its wastes am i right..?

Which statement is true for scyphozoan medusae?A.They are the dominant body form (I think it's this 1, am i right?)B.They are generally smaller than hydrozoan medusaeC.Most livein fresh waterD.They are scavengers